Electron discharge devices



July 22, 1958 Y M. E. HlNES 2,844,722 ELECTRON DISCHARGE DEVICES V FiledFeb. 4, 1955 r 2 Sheets-Sheet 1 FIG.

OUTPUT READ/Na SIGNAL 3? FIG. 2

v 4a (DIELECTRIC sun/14a) 4a 7 f 4/ )1" INPUT n'mrma SIGNAL 26: (BA PL86 IL X37 (BARR/ER GRID) I C/ l I I 6:1 J I 7 ourpur L L C23- g l -L- IR m 1 A 2 33 CANCELL/NG l 'PULSE GENERATOR yn/70R. M. E. -Hl/VESATTORNEY 2 Sheets-Sheet 2 CASE 3 FIG. 4

CASE 2 M. E. HINES ELECTRON DISCHARGE DEVICES CASE 1 INVENTOR M. E.H/A/ES BV dfiaqflz ATTORNEY ELECTRON DISCHARGE-DEVICES Marion E. Hines,Summit, N. J., assignor to B ell Tele- .phone Laboratories,Incorporated, New York, .N. Y.,

i a corporation of New York Application February 4,1955, Serial No.486,221 19 Claims. (Cl. 250-27 This invention relates to electrondischarge devices of the beam storage type and more particularly to thereading and writing of information in such devices.

/ One type of fast access short storage memory that hasbeen employed inrecent memory or storage systems utilizes electron discharge devices ofthe beam storage typeand particularly of the type known as a barriergridstorage tube. Such tubes are well known in the art,

'beingdescribed,for example, in an article Barrier-grid storage tube'andits'operation, by A. S. Jensen, J. P.

'Smith, M. H. Mesner, and L. E. Flory, R. CIA. Re-

view 'IX, p.112-135, March 1948, and in R. W. Sears 7 Patent 2,675,499,April 13, 1954.

- 2 lector efliciency. A third disadvantage in this type of operation isthat a number of secondary electrons are emitted whenever the beam isstriking the array, though temporarily greater or lesser amounts ofsecondaries may appear duringwriting or reading operations Theequilibrium value of secondary electron current at the collector forms apedestal upon which. the reading information is superimposed. Variationsin the fractional collection of'such secondary electrons aifect thepedestal also and consequently positive identification of the type ofinformation stored becomes more diflicult.

a concentrated electron stream against the other face of .theedielectricthrough a barrier grid which is positioned directly adjacent this otherface of the dielectric. In the operation of such a tube, the beam isdeflected intwo coordinate directions; for example, it may be repeatedlyswept in one direction and selectively'deflected in the other direction'orit may be turned on and deflected to a particular spot on thedielectric surface if completely raudom'access is desired. The operation,of the device involes, basically, two cycles, one store or write andthe other remove or read. During the writing cycle/the potential orcharge of elemental areas of the bombarded dielectric, surface is variedin accordance with an input signal, the'charge change being dependent onthe signal at the time the beam impinges on the area. the reading cycle,the charges upon these areas are re- During moved byaction of theelectron beam. Fundamentally the charging and discharging of theelemental areas above noted result from the emission of secondaryelectrons. The flow of secondary electron current can be detected inseveral ways and provides an output indication of the stored informationdurlng' the reading cycle. Generally the method ofoperation of the 7however. First, the signal received at the collector is-verysmall, notall of the emitted secondaries being collected at the collector.electrode.

Secondly, and more serious, the fractional amount of such collection isdependent on the position of origin in the storage array. Thus there arevariations in the size of the signal at the collector as a function ofspot position on the dielectric surface independent vof the storedinformation; This effect is called 'shading. Shading appears as adecrease in magnitude ofthe-reading signal as the spot approaches theedge By reading the stored information directly at the target structure,that is, by reading the signal caused by the secondary electrons leavingthe target instead of those arriving at the collector, the problem ofcollector efliciency and the variations in fractional amounts ofsecondary electrons collected are obviated. Further, this not onlyeliminates shading almost entirely but increasesthe magnitude of theoutput signal as well. However, it"is also most desirable to apply thewriting signal directly to the target assembly and thus in consideringobtaining the reading signal from the target assembly directly, it mustbe remembered that there are two signals that differ by at least threeorders of magnitude. This means that an amplifier connected to thetarget assembly for the purposeof amplifying reading signals must not bebadly jammed by the considerably larger writing signals.

It has been proposed 'to apply theinput signal to the barrier griddirectly and take the output signal from the back plate of the targetassembly, but with such an arrangement the writing. and reading, i. e.,the input and 7 read from the tube.

this inductance. However, when the information is be- 7 both conductorsof the line in the same direction; This of-the'-'target and is causedmostly by decrease in col- It is a further object of this inventiontoenable the reading and writing circuits both to be connected directlyto the target'assembly and yet tobe effectively and sub; stantiallycompletely isolated, electrically, from each other.

These and other objects of this invention are attained in one specificembodiment thereof wherein the Writing signal is applied directly to theback plate portion of the target assembly, as is preferred, and thereading signal is taken directly from the target assembly as a whole. Inthis specific embodiment, a coaxial line is connected through theenvelope of the tube so thatits inner cone ductor is connected'to theback plate and its outer conductor is connected to a shielding memberencompassing the target assembly, the barrier grid being supported bythe shielding member so that the outer conductor is in effect connected.to the barrier grid.

The coaxial line has a portion wound as an inductor'with the twoconductors having essentially equal in: ductances and a mutualinductance between the two 0on ductors equal to the self inductance ofeither. The .input or writing signal is then applied between these twoconductors. The'charging current requisite for the capacitance betweenthe barrier grid and the back plate ing in the two conductorseffectively cancel each other out in the coiled portion and induce novoltage across ing read, the surface of the dielectric is beingchargedjo'r discharged through capacitances to both the barrier grid andback plate simultaneously. so that current fiows in current produces anoutput signal voltage across the coiled portion of the coaxial linewhich can be detected and utilized by an output circuit.

The storage tube may be of the barrier-grid type, described above, ormay be of the dielectric-island type, disclosed in application SerialNo. 169,140, filed June 20, 1950, now U. S. Patent No. 2,726,328, issuedDecember 6, 1955, of A. M. Clogston. In a dielectric island tube aplurality of distinct small dielectric regions or islands are mounted ona back plate. A barrier grid is not employed but a field equalizing gridmay be advantageously located in front of the dielectric islands. In.embodiments of this invention utilizing such devices, the chargingcurrents are the sameas discussed above. However, here the condenserbeing discharged on reading of the stored information is substantiallysolely between the dielectric and the back plate so that the currentflows almost solely in the inner conductor of the coaxial line.

The reading signal may be detected directly across the coiled portion ofthe coaxial line, as by an amplifier connected thereto, or maybedetected by an amplifier connected to a transformer winding inductivelycoupled to the coiled portion. Further the writing signal issubstantially prevented from appearing across the coiled portion of theoutput-line by completely shielding the back plate and dielectricmembers of the target assembly, as by having the target assemblyelfectively encompassed by the outer conductor of the coaxial line andspecifically by a shielding member connected to the outer conductor andto which the barrier grid or field equalizing grid is attached. In thismanner, extraneous capacitances between target elements and ground orcollector, which may be at ground potential, are reduced or inhibitedthereby eliminating the charging current to these capacitances, whichcurrent would not be equally balanced in the two conductors of thecoaxial line.

It is a feature of this invention that the writing signal be applieddirectly to the back plate of the target assembly and the reading signalbe taken from the target assembly as a whole. More specifically, it is afeature of this invention that the writing and reading signals utilizethe same connectors to the target assembly.

It is another feature of this invention that a coaxial line be connectedto the target assembly of a beam storspecifically it is a feature ofthis invention that a shielding member connected to the outer conductorof the coaxial line encompass the back plate. Further in accordance withthis feature of the invention, the barrier or field equalizing grid maybe supported by the encompassing shielding member.

It is a feature of one specific embodiment of this invention that thereading signal be detected by an amplifier connected directly to thecoiled portion of the coaxial line. It is a feature of another specificillustrative embodiment of this invention that the reading signal bedetected by an amplifier inductively coupled to the coiled portion ofthe coaxial line. 7 V

It is still another feature of one specific embodimen of this inventionthat the coaxial line comprise a hollow metallic tube having a wirepositionedtherein, the metallic tube advantageously having adequatelythick walls to reduce the resistanceof the outer conductor of the line.Further it is a feature of this invention that the coiled portion of acopper tube be wound on amagnetic core.

A complete understanding of this invention'and of these .and variousother features thereof may be gained from consideration of the followingdetailed description and the accompanying drawing, in which:

Fig. 1 is a diagrammatic representation of one specific illustrativeembodiment of this invention;

Fig. 2 is a simplified circuit schematic for the reading and writingcircuits of the embodiment of Fig. 1;

Fig. 3 is a chart of currents and voltages for various conditions duringthe operation of the embodiment of Fig. 1; and

Fig. 4 is a diagrammatic representation of another specific illustrativeembodiment of this invention utilizing a dielectric island storage tube,only the target portion of the tube being shown.

Referring now to the drawing, Fig. 1 depicts an illustrative embodimentof this invention utilizing a barrier grid storage tube 10. As known inthe art, the tube 10 may advantageously comprise within an evacuatedenvelope, such as glass, an electron gun including a cathode 11, heater12, and accelerating and focusing electrodes 13, 14, and'15, defining anelectron lens, deflection plates 16 and 17, a collector electrode 18, ashield 19, and a target assembly 20. The target assembly .20 includes aback plate 22, a dielectric sheet 23, and a barrier grid 24, positioneddirectly in front of the dielectric sheet 23. In accordance with anaspect of this invention, the back plate 22 and dielectric sheet 23 areenclosed within a shielding member 26 to which the barrier grid isattached.

In storage tubes of this kind, information is stored by an electrostaticcharge on a discrete zone or area of the surface of the dielectric 23.To place such a negative charge on the surface, the electron beam isturned on while the back plate is temporarily raised to a positivepotential. This temporarily raises the potential of the front face ofthe dielectric through capacitive action. The electron beam then chargesthis surface withnegative electrons sufiiciently to drop its potential.to that of the barrier grid which is the equilibrium potential. -Duringthe charging operation, the secondary emission electrons from thedielectric return to it and cannot escape. When the beam is turnedelsewhere and the back plate potential returned to normal, the chargeremains, leaving the dielectric surface at a negative potential.

In this embodiment of the invention the writing circuit, which appliesthe positive Writing potential to the back plate during the storageoperation just described, comprises a coaxial line 28 having its innerconductor 29 connected to the back plate 22 and its outer conductor 30connected to the shielding member 26 and thus to the barrier grid 24.The coaxial line 28 has a portion 32 wound as an inductor with the twoconductors 29 and 30 having equal inductances at this portion and amutual inductance between them equal to the self-inductance of eitherone. A source 33 of input writing signals is connected between the innerand outer conductors 29 and 30 of the coaxial line 28.

When a writing signal is applied by the source 33, the current flowsalong the inner conductor 29 to charge the internal capacitance andreturns via the outer conductor. The coiled portion of the coaxial lineacts as a noninduc tive Winding, and, ideally, in the absence of beamcurrent I no voltage should appear between the barrier grid 24equilibrium, the dielectric surface releases as many electrons as arriveand remains at the barrier grid potential.

I It is thus apparent that when the information is being read out, thesurface of the dielectric is being discharged through capacitances bothto the barrier grid and the back the'coiled .portion 32 and an outputamplifier 35 is connected across the resistance 34 and the coiledportion 32.

Thus, application of the writing signal alone. to the back plateiideallywill induce no signal into the reading amplifier. However, the electronbeam will affect the reading amplifier by applying .current to thestorage target as a .whole. The net current to the target will be thedifference between the impinging beam current andv the escapingsecondary electron current. Any change in this net current will cause asignal to appear and be amplified.

Whenever the dielectric surface is being charged or discharged,whetherin reading or writing,there will bea deficiency or excess ofsecondary electron current escaping from the target; the detection andinterpretation of these current changes constitutes the readingoperation.

, It is an aspect of this invention to eliminate from the readingcircuit spurious signals induced from other sources than the electronbeam current, I -A schematic diagram of the reading and writing circuitsofthe embodiment of Fig. 1 is shown in Fig. 2. In

"this schematic diagram each conductor of the coiled portion 32 has aninductance L and a mutual inductance M, advantageously equal to L. Innerconductor 29 connects Ione inductance to the back plate, whichis'represented by the point36, and outer conductor 30connects the otherinductance t o-the barrier grid, which is represented by the point 37.Capacitance C which is the barrier grid to back plate capacitance, isconnected between points 36 and 37. C is the barrier grid to groundcapacitance, which is relatively large, and C is the back. plate toground capacitance, which is very small due to the shielding actionofthe shielding member 26 and the construction of the target assembly. Risthe equivalent resistance'of the-outer conductor of the coaxial line28 and-is also very small. a

Connected between the two inductors is thewriting signal source .33. Theoutput amplifier 35 -is connected across the load resistor 34 betweenthe barriergrid and ground. During the charging of the capacitor Cideally a slight unbalance of current in the two inductors.

When'the beam returns to the charged spot on the dielectric to read outthe information stored atthatspot, the charge on the dielectric isremoved and current flows to ground through both the input and outputconductors of the coaxial line. Wecan therefore consider that there is acurrent generator 40connected to a point 43 representing the dielectricsurface which is connected by a first capacitance 41,=representingthedielectric surface to back-plate capacitance,"to theback plate and by a second capacitance 42, representing the dielectricsurface to barrier grid capacitance, to the barrier grid. Thereadingrsignal source is thus-an assumed source 'ofcurrent that isactually the difference between the primary and secondary electroncurrents at the dielectric surface. As this source and its current pathare notactually i present, but only assumed, the source has beenindicated as connected in the circuit by dotted lines. v

The coiled portion 32 may advantageously be attained by coiling acoaxial cable, usingthe outer and'inner conductors as thegtwo selfinductances of an air core coil.

However, as depicted in 2, a magnetic core may also be employed. -Thusin one specific embodiment the coiled portion 32 comprises a wirepositioned within a copper tubing wound on a ferriteferromagnetic core.

f From the above discussion it is apparent that, when w the beam isturned on suddenly at a particular spot on the Y dielectric ,23, thereare four possible conditions to consider. These conditions are: a

Case 1: No charge'previously stored and no writing voltage applied;

Case 2: No charge previously storedand writing voltage app Case .3:Negative charge previously stored and n o writing voltage applied; and

Case 4: Negative charge prevlously stored and writing voltage applied. 11 In Fig. 3 the character of the current to and from the storage arrayfor each of these cases is shown as a function of time for a pulse ofsufficient duration to achieve equilibriumx Also shown are the forms ofthe voltage pulses generated in the reading circuit. In Case 1 only theequilibrium number of secondary electrons leave the surface. In Case 2writing of information on the spot is to occur and there is a'temporarydeficiency in the number of secondary electrons while the chargingoperation takes place with a gradual return to the equilibrium value. InCase 3 reading of the information priorly stored is to occur and anexcess number of secondary electrons is produced while the surface isdischarged, followed by a gradual return to'normal. There is also a netcur-rent which, as described above, flows inductively in the coiledportion 32 thereby inducing the output signal voltage. In Case 4 thecurrent is at an equilibrium value as in Case 1 because the positivewriting pulse alone should return the negatively charged surface to thepotentialof the barrier grid.

The writing signals applied to the target array are of .the order ofseveral tens of-volts while the reading signals received by theamplifier 35 are of the order of a few millivolts; the reading signalsmay then of course be amplified to any-desired level- In accordance withmy invention the application of the very large writing signals does notinterfere with the operation 'of the sensitive amplifier connected to.the reading circuit; Ideally, as discussed above, there should be nosignalat all across the coiled portion 32 on application of the Writingsignal. However, in effect a veryrsmall signal is induced due tounbalance in the currents flowingin the inner and outer conductors. Thisunbalance is,'-in part, caused by other capacitances within the tube. Itis therefore desirable that the back plate anddielectric besubstantially entirely shielded from'the rest .of the tube. Accordinglyin the embodiment of the invention depicted in Fig. l, the shield member26, which is an extension of the outer conductor 30, completelyencompasses these elements.

However, there is a very small direct capacitance between the back plateand the other elements in the. tube, specifically'the shield 19 andcollector 18, through the holes in the barrier grid. The shield 19 isadvantageously grounded and the collector 18 maybe connected directly toground, asby being connected to the shield 19, or be connected toground. through. a resistance, as shown in Fig. 1'. This capacitance canbe considered, therefore, as a back plate-to-ground. capacitance, and isthe capacitance 63in Fig. 2. Accordingly, not quite all the chargwhichprevents the perfect current balance desired is' caused by the finiteres'ista'nce of the outer conductor of the line. The charging currentwill therefore induce a slight voltage between the barrier grid andground. This voltage, however, can be kept quite small,.well below themagnitude of thereadiug output signals, so that discrimi- -nationbetween the twotypes of induced signals is not wherein a canceling pulsegenerator 45 is operated by the writing pulse source 33 to apply acanceling pulse of proper sign and magnitude directly to the readingamplifier 35, as directly across the load resistor 34 'to cancel out theresidual error signalappearing at that resistor due to the unbalance ofcurrent flow in the two conductors of the coaxial line.

In Fig. 4 there is shown another specific illustrative embodiment ofthis invention wherein the storage tube comprises a dielectric islandtube of the type disclosed in application Serial No. 169,140, filed June20, 1950, now U. S. Patent No. 2,726,328, issued December 6,

1955, of A. M. Clogston. In the embodiment of Fig. 4, the target arraycomprises a back plate 50 on the front surface of which are locatedsmall spots or islands 51 of dielectric material. A field equalizinggrid 52 is positioned in front of the dielectric islands 51 and betweenit and the remainder of the tube, which may be as depicted in Fig. 1;grid 52 is supported by the shield member 26 encompassing the targetarray.

In this specific embodiment the'output amplifier 35 is not directlyconnected across the coiled portion 32 but is coupled thereto by aninductive winding 55, whereby transformer coupling is attained with anincrease in signal strength over the direct coupling method of 'Fig. 1.

It is to be understood that the above-described arrangements areillustrative of the application of the principles of the invention.Numerous other arrangements may be devised by those skilled in the artwithout departing from the spirit and scope of the invention.

What is claimed is:

1. An electron discharge device comprising a back plate, dielectrictarget means in contact with said back plate, an electrode positionedtothe other side of said dielectric means than said back plate, electrongun means for projecting a beam-of electrons through said electrode andagainst said dielectric means, and means comprising said gun means, saidback plate and said electrode for storing information on said dielectricmeans and for receiving stored information from said dielectric means,said last-mentioned means further comprising an intermittent voltagesource and a coaxial'line connected to said source and having its innerand outer conductors individually connected to said back plate and saidelectrode.

2. An electron discharge device comprising a back plate, .dielectrictarget means mounted on said back plate, a grid positioned in front ofsaid dielectric means, electron gun means for projecting a beam ofelectrons through said grid and against said dielectric means, and meansincluding said electron gun means for applying signals to said backplate to store information on said dielectric means and for receivingoutput information from said dielectric means, said last-mentioned meansincluding a coaxial line having its inner conductor connected to saidback plate and its outer conductor connected to said grid, said coaxialline having a coiled portion, a source of intermittent input signalsconnected between .said inner and outer conductors, and means coupled tosaid coiled portion for receiving output signal voltages developedthereacross.

3. An electron discharge device in accordance with claim 2 furthercomprising a shielding member encompassingsaid back plate and saiddielectric means, said shielding-member ,being connected to said outerconductor of said coaxial line.

4. An electron discharge device in accordance with claim 3 wherein saidgrid is mounted by said shielding member.

*wirepositioned within a hollow conducting tube.

7. An electron discharge device in accordance with claim 6 wherein saidcoiled portion encompasses a magnetic core.

8. An electron discharge device in accordance with claim 2 wherein saiddielectric target means comprises a dielectric sheet and said grid is abarrier grid positioned closely adjacent said dielectric sheet.

9. An electron discharge device in accordance with claim 2 wherein saiddielectric target means comprises a plurality of distinct dielectricislands mounted on said back plate.

10. An electron discharge device in accordance with claim 2 wherein saidmeans coupled to said coiled portion for receiving output signalvoltages comprises an amplifier connected directly to said coiledportion.

11. An electron discharge device in accordance with claim 2 wherein saidmeans coupled to said coiled portion for receiving output signalvoltages comprises an amplifier inductively coupled to said coiledportion.

12. An electron discharge device comprising a back plate, dielectrictarget means mounted on said back plate,

an electrode positioned to the other side of said dielectric'means thansaid back plate, a shielding member encompassing said back plate anddielectric means and connected to said electrode, electron gun means forprojecting a stream of electrons against said dielectric means, andmeans including said electron gun means for applying signals to saidback plate to store information on said dielectric means and forreceiving information from said dielectric means, said last-mentionedmeans including a first conductor connected to said back plate and asecond conductor connected to said shielding member, said conductorstogether defining a coiled portion, and a source of intermittent inputsignals connected between said first and second conductors.

13. An electron discharge device in accordance with claim 12 furthercomprising means coupled to said coiled portion for receiving outputsignal voltages developed thereacross.

14. An electron discharge device comprising a back plate, a dielectrictarget means mounted on said back plate, a grid positioned in front ofsaid dielectric means, a shielding member encompassing said back plateand dielectric means and mounting said grid, electron gun means forprojecting a stream of electrons through said grid against saiddielectric means, and means including said electron gun means forapplying signals to said back plate to store information on saiddielectric means and for receiving information from said dielectricmeans, said lastmentioned means including a coaxial line having itsinner conductor connected to said back plate and its outer conductor tosaid shielding member, said coaxial line comprising a wire within ahollow metallic tubing and having a coiled portion, input signal meansconnected between said inner and outer conductors, and means coupled tosaid coiled portion for receiving output signal voltages developedthereacross.

15. An electron discharge device comprising a back plate, a dielectrictarget means in contact with said back plate, a grid positioned in frontof said dielectric means, electron gun means for projecting a stream ofelectrons through said grid and against said dielectric means, and meansincluding said electron gun means, said back plate and said grid forstoring information on said dielectric means and for receiving storedinformation from said dielectric means, said last-mentioned'meansincluding a coaxial line having its inner and outer conductorsindividually connected to said back plate and said grid and output meanscoupled to said coaxial line for receiving an output signal on flow ofcurrent in only one direction in said coaxial line but no output signalon flow of current in opposite. directions in said inner and outerconductors of said coaxial line.

16. An electron discharge device comprising a back plate, dielectrictarget means in contact with said back plate, a grid positioned in frontof said dielectric means,

a shielding member encompassing said plate and dielecon said dielectricmeans and for receiving information from said dielectric means, saidlast-mentioned means further comprising a coaxial line having its innerconductor connected to 'said back' plate and its outer conductor to saidshielding member, a source of writing signals connected between saidinner and outerconductors, an output circuit, and means preventing anoutput signal being applied to said output circuit on application ofsaid writing signals to said coaxial line.

17. An electron discharge device in accordance with claim 16 whereinsaid output circuit is electrically connected to said preventing means,the output signals being across said preventing means on the reading ofinformation stored on said dielectric means.

18. An electron discharge device comprising a back plate, dielectrictarget means in contact with said back plate, an electrode positioned tothe other side of said dielectric means than said back plate, electrongun means for projecting a stream of electrons through said electrodeand against said dielectric means and means including said electron .gunmeans, said back plate and said electrode for storing information onsaid dielectric means and for receiving stored information from saiddielectric means, said last-mentioned means including a first conductorconnected to said back plate, a second conductor adjacent saidfirstconductor so as to be electromagnetically coupled thereto andconnected to said electrode, a source of intermittent writing signalsconnected between said first and second conductors, and output meanscoupled to said conductors for receiving an output signal on flow ofcurrent in only one direction in said conductors but no output signal onflow of cur-rent in opposite directions in said conductors.

19. An electron discharge device comprising a back plate, dielectrictarget means in contact with said back plate, a grid positioned in frontof said dielectric means,

a shielding member encompassing said plate and said dielectric means andconnected to said grid, electron gun means for projecting a stream ofelectrons against said dielectric means, and means including saidelectron gun means, said back plate, and said grid for storinginformation on said dielectric means and for receiving information fromsaid dielectric means, said last-mentioned means including a firstconductor connected to said back plate, a second conductorelectromagnetically coupled to said first conductor and connected tosaid shielding member, a source of intermittent writing signalsconnected between 'said conductors, an output circuit, and meanspreventing an output signal being applied to said output circuit on flowof writing current in both said conductors.

References Cited in the file of this patent UNITED STATES PATENTS2,097,491 Laie et al. Nov. 2, 1937 2,251,573 Morton Aug. 5, 19412,257,795 Gray Oct. 7, 1941 2,419,907 Mole Apr. 29, 1947 2,420,846Strutt et al. May 20, 1947 2,675,499 Sears Apr. 13, 1954 2,700,151'Flory Ian. 18, 1955 OTHER REFERENCES The Bell System Technical Journal,vol. 34 pp. 1241 to 1264, November 1955, Digital Memory in Barrier- GridStorage Tu'bes, M. E. Hines et al.

